If $x \oplus y = x-7y$ and $x \veebar y = 4x-4y$, find $(0 \veebar 0) \oplus -4$.
First, find $0 \veebar 0$ $ 0 \veebar 0 = (4)(0)-(4)(0)$ $ \hphantom{0 \veebar 0} = 0$ Now, find $0 \oplus -4$ $ 0 \oplus -4 = 0-(7)(-4)$ $ \hphantom{0 \oplus -4} = 28$.